I have an idea, and I have tried and happy results.

Normally a servo draws 250 to 300mA current from Rx battery pack, and if there are 4 servos (out of which 2 are constantly used, ailerons and elevator and 1 partially used, throttle, and rudder is used under certain conditions), so an average current of 800mA.

Also the servo motor draws maximum curent during rotation or initial start and average current during loading.

Now the point over here is that almost all of us have computer radios, and we can control the travel throw of servos. Now if we reduce the throw and put the push rod down to 1 or 2 holes in the control horn (this will not effect the sensitivity) then the servos will have to move less and correspondingly will draw less current on the average and this inceases the flight time (Rx battery will serve 2 to 3 more flights) .